1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to position control of an image which is formed on a sheet. More particularly, the invention relates to an image forming apparatus which can align image forming positions at high precision when a sheet is processed.
2. Description of the Related Art
Hitherto, in an image forming apparatus such as a copying apparatus, a laser beam printer, a facsimile, hybrid apparatus (a multi function apparatus) of them, or the like, a surface of a photosensitive drum is exposed by an exposing device, a latent image is formed onto the photosensitive drum, and thereafter, the latent image is developed, thereby forming a toner image. Further, the toner image is transferred onto a sheet and, thereafter, the transferred toner image is fixed onto the sheet by a fixing device.
In such a conventional image forming apparatus, there is an apparatus in which when the toner image is transferred, an image forming (transferring) position is corrected in accordance with a position in the sheet width direction which perpendicularly crosses the sheet conveying direction for the photosensitive drum so as to transfer the toner image to a predetermined position of the sheet.
FIG. 13 shows a construction of an image forming portion of such a conventional image forming apparatus which can correct the image forming position. When sheets Sa and Sb are conveyed for a photosensitive drum 300 so as to have edge positions X and X′, respectively, the start positions on which images are started writing onto the photosensitive drum 300 by writing units 3 and 4 are controlled so as to change them in correspondence to the edge positions X and X′ of the sheets Sa and Sb. Such control is made by automatic writing position deciding means.
According to Japanese Patent Application Laid-Open No. 2001-125440, in the image forming apparatus having such automatic writing position deciding means, a deviation of a sheet S is detected by a plurality of detection elements and a position of image data in a memory is corrected on the basis of a detection result. According to the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2001-125440, the deviation is detected at a detection resolution of 0.25 mm or higher and is corrected, thereby enabling the high-precision deviation correction to be realized by making the most of a writing position control ability which the digital writing units 3 and 4 having laser light emitting elements have.
In the conventional image forming apparatus, there is also an apparatus in which after the image forming position to the sheet was corrected at the high precision by the automatic writing position deciding means, in order to execute a process such as binding process, punching process, or the like to a sheet bundle in which a predetermined number of sheets have been stacked, a sheet processing apparatus is provided.
In such a sheet processing apparatus, each time the sheets on which images have been formed are ejected onto a processing tray one by one, a position of the sheet in the ejecting direction is aligned and a position of the sheet in the width direction is aligned by using an aligning plate. Further, as such a sheet processing apparatus, Japanese Patent Application Laid-Open No. H10-181981 discloses an apparatus in which a pair of aligning plates which can independently be driven are provided and when the position of the sheet in the width direction is aligned, the sheet is aligned while changing the aligning position by using the pair of aligning plates in accordance with a processing mode such as a stapling process or the like.
In the conventional image forming apparatus in which the image forming position is corrected in accordance with the position of the sheet in the width direction before the image is formed, the image forming position to the sheet can be controlled at the high precision. However, the sheet obtained after the image creation is expanded and contracted by heating and pressurization when the sheet passes through the fixing device. Further, an expansion/contraction amount differs depending on a toner amount of the image transferred to the sheet.
In the case where the sheets have been expanded or contracted as mentioned above, when the positions of the sheets in the width direction are aligned, since the sheets become irregular in the width direction, the edge portion of an image area formed on each sheet is also deviated.
Further, in the case where a bundle of sheets to be processed is constructed by sheets of different materials such as partition paper, cover, back cover, and the like, there is such a problem that a sheet irregular amount increases and the image forming area edge portion is also further deviated.
When the sheets as mentioned above are aligned and staple-processed as a sheet bundle, since they are bound in the sheet irregular state, there is such a problem that even if the image forming area edge portions from the edge portions of the sheets are aligned at high precision, an advantage as a product is not effected.